Sybil's mother is an aged actress who presumably had Sybil after having an affair with an upper-class man who was married. In her demeanor, as well as in her speech (as she is characterized directly and indirectly), she denotes a penchant for connecting Dorian to the aristocracy. Being that she considers Sybil to be made of different stock due to her father possibly being an aristocrat himself, she finds it more than possible to make a connection; she tends to forget that, regardless of what kind of blood ran through Sybil's veins, she is still a fatherless woman with no dowry, and with very little prospects. However, this does not detain Sybil's mother from dreaming away
Of course, if this gentleman is wealthy, there is no reason why she should not contract an alliance with him. I trust he is one of the aristocracy. He has all the appearance of it, I must say. It might be a most brilliant marriage for Sibyl. They would make a charming couple. His good looks are really quite remarkable; everybody notices them.
Contrastingly, James Vane, Sybil's sister, does not trust the upper-classes. He only knows about the possible affair of his mother through gossip, and when he confronts her about it he feels that this could be a predictor of Sybil's own future. For this reason, Vane prefers to keep a close eye on Sybil and declare war on whoever hurts her.
The way in which both views of Dorian atone for Lord Henry's thoughts on the matter is that they coincide with the fact that Dorian has manipulative ways, and plenty of tools to get away with just about anything. And he eventually does.